It is known to improve the color and transparency to light of plastics, especially of aromatic polycarbonates, by adding neutral esters of phosphorous acid. Esters of phosphorous acid with alcohols containing oxetane groups have proved particularly suitable. In contrast to phosphites used previously, these phosphites prevent the severe brown discoloration of the plastic under the influence of atmospheric oxygen and heat. The phosphites containing oxetanes, the use of which is described in DT-OS (German Published Specification) No. 2,140,207, and U.S. Pat. No. 3,794,629 can be prepared in a known manner by transesterification of trialkyl phosphites or triaryl phosphites with oxetane compounds of the type ##STR1## wherein
Z = H, alkyl, cycloalkyl or aryl, (compare U.S. Pat. No. 3,209,013).
The invention is based on the recognition of the fact that it is not necessary to employ phosphites containing oxetane groups and that instead the addition of even small amounts of a mixture of trialkyl phosphites or triaryl phosphites and oxetane compounds, preferably the alcohols containing oxetane groups which are also used to prepare oxetane-containing phosphites, to the plastic produces an equally good lightening of the color, and that the plastics stabilized in this way also prove to be highly stable in respect of color, and in some cases even more stable, in respect to color, to heat and atmospheric oxygen.
It is furthermore known to employ phosphites, mixed with epoxide compounds, as color lighteners and stabilizers in polycarbonates (DT-OS (German Published Specifications) Nos. 1,694,285 and 2,040,251). However, these mixtures are far inferior to the mixtures of phosphite with oxetane compound.